A semiconductor laser is used as a light source in an opto-electronics field such as an optical disc device, optical fiber communication, laser printer and optical measuring. The optical disc device which uses a compact disc or digital versatile disc as a recording/playback medium, for example, brings out the feature of the semiconductor laser emitting light with the same wavelength and phase to read (play back) and write (record) data on/to the disc by the light converged to the wavelength size by a lens.
In the semiconductor laser for the playback, characteristics such as wavelength spectrum, far-field image in horizontal/vertical directions, noise, astigmatic difference, polarization and reliability in addition to warranty of actuation in a wide temperature range are required so as to satisfy specifications of an optical pickup head. In these characteristics, the noise characteristic is one of the important characteristics to determine the quality of the playback signal. As the main noises made in the semiconductor laser, in a longitudinal single mode actuation, there are modal competition noise caused by change in temperature and electrical current and optical regression noise caused by re-incidence of regression light from a laser end face. For the optical disc device, suppression of the optical regression noise is especially important because reflections from the optical disc and collimator lens generate the regression light.
In order to decrease the optical regression noise, it is required to weaken coherence of the laser by changing the longitudinal mode to a multi-mode and making the wavelength chirping by causing variation of refraction index in an active layer. As a method for this purpose, it is known that the laser is oscillated at high speed pulses to a current lower than an oscillation threshold by superposing a high-frequency current of hundreds MHz to several GHz on a direct current for driving the laser. According to this method, a high-frequency oscillator which is separately necessary increases the cost. The high-frequency current also generates EMI (Electro-Magnetic Interference). If parts for countermeasure against the EMI are provided, the cost is further increased.
On the other hand, as another art to suppress the optical regression noise, there is a method using “a self-pulsating semiconductor laser”. In the self-pulsating semiconductor laser, a region referred to as “a saturable absorber” is provided around an active layer. The saturable absorber which has a switchable function between absorption and transmission of the laser causes variation in the refractive index in the active layer because the amount of optical loss in the saturable absorber is linked to the change in the number of carriers in the active layer. The variation of the refractive index in the active layer is an important factor for the generation of the wavelength chirping and changes difference Δn between the refractive indexes inside and outside, with respect to a direction a parallel to a joint surface of the diode, of a waveguide in a direction vertical to the axial direction of a resonator. A series of actuations automatically takes place in the semiconductor device driven by the direct current, the intensity of the laser automatically changes in the frequency range from hundreds MHz to several GHz, and therefore the same effect as the superposition of the high-frequency current can be obtained in a single element.
The self-pulsating semiconductor laser is disclosed in Patent Documents 1 and 2, for example.
The semiconductor laser described in Patent Document 1 has a double hetero-structure having an active layer of GaInP or AlGaInP on a GaAs substrate of a first conductive type and cladding layers of AlGaInP which are provided on both sides of the active layer and have a refractive index lower than that of the active layer. The cladding layer of a second conductive type over the active layer has a mesa structure on the top surface of the active layer. The semiconductor laser has a (AlxGa1-x)0.5In0.5P layer of the second conductive type on the side faces of the mesa structure and on the top surface of the active layer on both sides of the mesa structure and further has a GaAs layer on the (AlxGa1-x)0.5In0.5P layer of the second conductive type.
The self-pulsating semiconductor laser described in Patent Document 2 has a semiconductor substrate of a first conductive type and a multilayered structure including an active layer and provided on the semiconductor substrate. The multilayered structure includes a first cladding layer of the first conductive type provided under the active layer, a second cladding layer of a second conductive type having a striped ridge portion and provided above the active layer, and a saturable absorbing film provided on the second cladding layer, the saturable absorbing film having an accumulation region for accumulating photoexcited carriers, the accumulation region being provided apart from a surface of the second cladding layer.    [Patent Document 1]    JP Patent Kokai Publication No. JP-A-04-154184    [Patent Document 2]    JP Patent Kokai Publication No. JP-A-09-181389